Carbon−Carbon Bonds Functioning as Electron Shuttles:  The Generation of Electron-Rich Manganese(II)−Schiff Base Complexes and Their Redox Chemistry

The reduction of [Mn(II)-salophen] derivatives [salophen = N,N‘-ethylenebis(salicylideneaminato) dianion] led to the formation of C−C bridged dimers. Such C−C bonds function as two electron shuttles in electron-transfer reactions. The reduction of [Mn(salophen)(THF)]2 (1) and [Mn(3,5-But4salophen)(THF)]2 (3) with 2 equiv of sodium metal led to the corresponding single C−C bond bridged dimers, [Mn2(salophen2)Na2(DME)4] (5) [salophen2 = C−C bonded salophen dimer] and [Mn2(3,5-But4salophen2)Na2(DME)6] (7), respectively. Complexes 5 and 7 undergo a further two electron reduction to [Mn2(*salophen2*)Na4(DME)6] (6) [*salophen2* = C−C doubly bonded salophen dimer] and [Mn2(*3,5-But4salophen2*)Na4(DME)4] (8), respectively, both containing a double C−C bridge. The obtention of [Mn2{salophen(Me)CH2)}2Na4(DME)4] (9) from [Mn(salophen-Me2)(THF)]2 (2) strongly supports the existence of free radical precursors in the formation of C−C bonds. Complex 6 has been used as a source of four electrons in a number of reactions, thus reduction of Ag+, PhCH2Cl, p-benzoquinone, and [CoII(MeOsalen)] occurs with the regeneration of the starting material 1. The C−C bond cleavage is the source of electrons, without being involved in any reaction as a reactive site. With stronger oxidizing agents not only complexes 6 and 8 transfer the electrons stored at the C−C bonds but also the metal undergoes a change in the oxidation state. The reaction of 6 with dioxygen produces a novel form of di-μ-oxo-Mn(IV) dimers, where the salophen ligand displays a bridging bonding mode in [Mn2(μ-salophen)2(μ-O)2] (15) and [Mn2(μ-3,5-Butsalophen)2(μ-O)2] (16).